In 3GPP terminology a higher layer protocol delivers a Packet Data Unit (PDU) to a lower layer protocol and expects it to be delivered to its peer entity. In other words, the lower layer provides a transmission service to the higher layer. From the lower layer's perspective the higher layer data unit is therefore the service data unit (SDU). A protocol layer (such as the RLC layer in LTE or WCDMA) may segment SDUs if the entire SDU cannot be transmitted at once. Furthermore, it may concatenate multiple such (segments of) SDUs. Finally, one or more such (segments of) SDUs are encapsulated into one PDU. A header is created that contains information about how to interpret the payload section of the PDU. The PDU is then delivered to the next lower layer which handles it as an SDU.
FIG. 7 shows the header of a typical MAC PDU. It may comprise one or more RLC PDUs (also referred to as MAC SDUs) and each of those may comprise (segments of) one or more RLC SDUs (e.g. PDCP PDUs). Such a MAC PDU is transmitted by the lower layers towards the peer protocol stack. Based on the header information the MAC- and RLC-Receiver can reassemble the original RLC SDU.
Of particular interest for this invention are the Length Fields and Length Indicators in the MAC and RLC header. They provide the receiving peer protocol entity with necessary information to reassemble the contained SDUs.
In the context of this invention the Length Field in the MAC header and the Length Indicator in the RLC header specify the size of a service data unit (SDU) or a segment thereof. Alternatively, the fields could provide information about the position of a SDU, i.e., the distance of the first byte of an SDU from the first (payload) byte of the containing PDU. The former is however more efficient if the largest supported SDU is smaller than the largest supported PDU. Both types of fields allow the receiver to determine the size and thereby position of the first and last byte of any contained SDU or segment thereof. In the remainder of this document we therefore use the term SDU position identifier as a generic term for such fields.
The required size of the SDU position identifiers depends on the expected size and size-granularity of the addressed SDUs and/or the containing PDU. A commonly used granularity is one byte. In order to be able to address any size or position inside PDUs of up to 2x byte the Length Fields and Length Indicators must comprise at least x bit (e.g. 215=32768). x may be the size of the largest supported SDU if that is known to be smaller than the largest supported PDU.
In state of the art protocols the size of such length and position identifiers are statically or semi-statically configured based on the maximum expected PDU or SDU size.
FIG. 7 illustrates an example of an RLC/MAC header and gives an estimate of the number of bits required for the header fields in LTE RLC and MAC. It can be seen that the Length Field (LF) as well as the Length Indicator (LI) contribute significantly to the overall size of the header. As long as the payload is large compared to the size of the header this is not a big problem. However, if only a few bytes of payload need to be encapsulated, this is a waste of resources.
If the payload in a PDU is small the relative header overhead is large. If e.g. only a single VoIP packet is included in one RLC PDU which is then encapsulated in one MAC PDU the Length Field (LF) generates a considerable overhead. The reason is that a fixed size Length Field must also support very large transport blocks so that e.g. 15 bit must be used. For small packets of e.g. 30 Byte this accumulates to an overhead of ˜6%. As the majority of all SDUs are small packets (e.g. VoIP, TCP-ACKs, and SIP Signaling) this becomes significant.
Another problem occurs if several small SDUs are to be encapsulated in a single RLC PDU. Then, a Length Indicator (LI) is required for all but the last one as shown in FIG. 8. If each of the embedded RLC SDUs is e.g. only 30 byte (240 bit) than the overhead of 13+1 bit equals to almost 6% for all but the last RLC SDU.
FIG. 8 illustrates a RLC/MAC header of an RLC/MAC PDU with 4 RLC SDUs encapsulated in one RLC PDU.
The UMTS RLC protocol as described in for instance 3GPP TS 25.322 already gives the possibility to adapt the size of the Length Indicators to the maximum expected PDU size. However, in UMTS RLC the PDU size is pre-configured (for UM the maximum size is pre-configured and the LI depends on this maximum size).
EP 1104207 discloses a data packet with a variable payload field and where the solution changes a length indicator field to the appropriate length of fixed granularity. In order to accomplish this, a separate field is added in the header to indicate if the length of the payload is not an integral unit of octets and if so the appropriate granularity. This has the disadvantage of requiring an examination at each level of infrastructure network of each packet in order to deduce the size and type of data packet. It requires changes of the standardization implementation of data packets and does still require considerable header size.
Furthermore, only two different LI-sizes are supported in UMTS.